There is a formula that shows how much water is produced from burning 1 gallon of pertrol, I think it something like 1.5 gallons of water per gallon of petrol burnt.
I guess that in cold climates (or winter) we see more condensation as the colder air can't hold the water vapour so its visible where in warm climates (or summer) its contained so not visible.
I'm sure someone with a PHD in physics can explain whats happening and why.
